A Systematic And Mechanistic Analysis Of The Water-Gas Shift Reaction Kinetics On Low And High Temperature Shift Catalysts CA Callaghan, I Fishtik, and R Datta Fuel Cell Center Department of Chemical Engineering Worcester Polytechnic Institute Worcester, MA 01609-2280, USA

introduction and motivation Fuel Cells H2 Infrastructure Fuel Processing Water-Gas Shift Better understanding of mechanism & kinetics of reforming reactions (e.g. WGSR) More systematic design of reformers and their catalysts

the WGSR mechanism a – activation energies in kcal/mol (θ  0 limit) estimated according to Shustorovich & Sellers (1998) and coinciding with the estimations made in Ovesen, et al. (1996); pre-exponential factors from Dumesic, et al. (1993). b – pre-exponential factors adjusted so as to fit the thermodynamics of the overall reaction; The units of the pre-exponential factors are Pa-1s-1 for adsorption/desorption reactions and s-1 for surface reactions.

reaction route graphs and topology Full Routes (FRs): a RR in which the desired OR is produced Empty Routes (ERs): a RR in which a zero OR is produced (a cycle) Intermediate Nodes (INs): a node including ONLY the elementary reaction steps Terminal Nodes (TNs): a node including the OR in addition to the elementary reaction steps Stop Start A RR graph may be viewed as several hikes through a mountain range: Valleys are the energy levels of reactants and products Elementary reaction is a hike from one valley to adjacent valley Trek over a mountain pass represents overcoming the energy barrier Ref. Fishtik, I., C. A. Callaghan, et al. (2004). J. Phys. Chem. B 108: 5671-5682. Fishtik, I., C. A. Callaghan, et al. (2004). J. Phys. Chem. B 108: 5683-5697. Fishtik, I., C. A. Callaghan, et al. (2005). J. Phys. Chem. B 109: 2710-2722.

the RR graph of the WGSR OR s1 s2 s14 s10 s3 s5 s13 s15 s11 s8 s6 s7

the electrical analogy Kirchhoff’s Current Law Analogous to conservation of mass Kirchhoff’s Voltage Law thermodynamic consistency Ohm’s Law Viewed in terms of the De Donder Relation a b c d e f g i h

simplified RR network We may eliminate s13 and s16 from the RR graph; they are not kinetically significant steps This results in TWO symmetric sub-graphs; we only need one

overview of analysis, reduction and simplification procedure R4 + R6 vs. R14 Effect of R14 on Conversion Experimental Conditions: Space time = 1.80 s FEED: COinlet = 0.10; H2Oinlet = 0.10 CO2 inlet = 0.00; H2 inlet = 0.00 On Cu(111)

the reduced network Cu(111) Fe(110)

the mechanism CO H2O s1 s2 s6 s8 s4 CO2 s3 H2 s5 s10 s15 s11 s7 s9 Formate RR CO2 s3 H2 s5 s10 s15 Modified Redox RR Associative RR s11 s7 C O H s9

the reduced rate expression Cu(111) Fe(110) Experimental Conditions: Space time = 1.80 s FEED: COinlet = 0.10; H2Oinlet = 0.10 CO2 inlet = 0.00; H2 inlet = 0.00

Energy diagram Cu(111)

conclusions and acknowledgements Reaction network analysis is a useful tool for reduction, simplification and rationalization of the microkinetic model. Reaction stoichiometry translates into the network connectivity (i.e. IN, TN) Allows for a more systematic approach for the analysis of microkinetic mechanisms. Analogy between a reaction network and electrical network exists: rate = current affinity = voltage resistance = affinity/rate Application of RR graph theory to the analysis of the WGS reaction mechanism validated the reduced model and confirmed earlier results* based solely on a conventional microkinetic analysis. A reliable predictive microkinetic model for the WGS reaction on Cu(111) and Fe(110) is developed Only a limited number of RRs dominate the kinetics of the process Prediction of simplified models compare extremely well with the complete microkinetic model. Funding: Office of Naval Research * Callaghan, C. A., I. Fishtik, et al. (2003). Surf. Sci. 541: 21.

topological characteristics Full Reaction Routes FR1: OR = s1 + s2 + s3 + s4 + s5 + s6 + s10 FR2: OR = s1 + s2 + s3 + s4 + s5 + s6 + s7 + s9 FR3: OR = s1 + s2 + s3 + s4 + s5 + s6 + s8 + s11 FR4: OR = s1 + s2 + s3 + s5 + s6 + s7 + s15 FR5: OR = s1 + s2 + s3 + s5 + s6 + s7 + s9 - s11 + s17 Empty Reaction Routes ER1: 0 = -s4 - s6 + s14 ER2: 0 = -s4 - s9 + s15 ER3: 0 = -s8 + s10 - s11 ER4: 0 = -s4 - s11 + s12 + s15 ER5: 0 = -s4 + s8 - s10 + s17 Intermediate Nodes IN1: r2 - r6 - r13 - r14 + r16 IN2: r1 - r7 - r8 - r10 IN3: -r3 + r7 + r10 + r11 + r12 + r16 +r17 IN4: r4 - r5 + r14 + r15 + r17 IN5: r6 - r8 - r9 - r10 + r12 + 2r13 + r14 - r15 - r16 Terminal Nodes TN1: -s9 - s10 - s11 + s13 - s15 - s16 - s17 + OR TN2: s8 - s11 - s12 - s16 - s17 + OR TN3: -s7 - s10 - s11 - s12 - s16 - s17 + OR TN4: s6 + s13 + s14 - s16 + OR TN5: -s5 + OR Example: the water gas shift reaction